The present invention relates to an encoding method for an anti-collision system for sea navigation.
A known anti-collision system for sea navigation is described in the French patent application No. 2 601 168. Each ship fitted out with this system has a transmitter repetitively transmitting a message containing, in particular, information on the geographical coordinates, the speed and the course of its own ship, and a receiver connected to the display device displaying, in particular in the form of symbols, homologous information received from other similarly equipped ships.
Such a system greatly facilitates the maneuvers performed by ships, especially in zones with a high density of obstacles (other ships, buoys, jetties etc.).
This known system also provides for the inclusion, in the transmitted messages, of the identification code of the ship with which communication is to be undertaken. This ship is automatically alerted as soon as its receiver picks up the message. The operators of these two ships can then communicate by telephone links, for example to coordinate their maneuvers. However, a telephone link such as this has drawbacks: the radio-frequencies available for such links are often very busy and even saturated. Moreover, operators rarely speak the same language.
Besides, in this known system, the messages also include the identity of the sender ship. The identity code is a sequence of binary elements, one part of which is used for the definition of the type of ship (petrol tanker, ferry, cargo ship etc.) while the other part is used to identify the sender ship without ambiguity. It is enough that this other part should be long enough for one and only one user ship in the world to be made to correspond to its binary content. It is enough to have a glossary to trace the code back to its corresponding user.
However, a possibility of biunique correspondence such as this between a code and a user may have one drawback. For a variety of reasons, (confidentiality, business competition, etc.), certain potential users of the system would not like their whereabouts to be known. The above-mentioned identification method may induce them to refrain from using their instruments, thus depriving other nearby ships of the advantages of the system. Moreover, the complexity of the task of preparing a glossary on a worldwide scale may be so cumbersome as to hamper the adoption of the system.